The wafer cleaning step in a very large scale integrated circuit (VLSI) production process serves for removal of resist residue, fine particles, metal, oxide films formed spontaneously, etc., and generally uses a mixed solution of concentrated sulfuric acid and hydrogen peroxide (SPM) or a mixed solution produced by blowing ozone into concentrated sulfuric acid (SOM). When hydrogen peroxide or ozone is added to high-concentration sulfuric acid, the sulfuric acid is oxidized to form persulfuric acid. It is known that persulfuric acid shows a high oxidizing power and high cleaning performance as it self-decomposes, and can serve effectively for cleaning wafers etc.
In addition to the above-mentioned persulfuric acid production methods, it is known that a persulfuric acid solution for cleaning can be produced by electrolyzing an aqueous solution containing sulfate ions in an electrolysis reactor (see patent references 1 and 2).
[Patent reference 1] Tokukai (unexamined Japanese patent publication) 2001-192874
[Patent reference 2] Tokuhyo (published Japanese translation of PCT publication for patent application) 2003-511555